1. Field of the Invention
The present invention relates to a rotation angle detecting device for detecting a rotation angle of a rotation shaft by detecting a rotation of a magnetic field vector due to a rotation of the rotation shaft.
2. Description of the Related Art
In a conventional steering angle detecting device, a rotation angle detecting device detects a change in a rotation angle of a magnet including a magnetized body using a magnetic sensor element. US 2005/0030011 A (corresponding to JP-A-2005-3625) discloses a rotation angle detecting device that can detect a rotation angle of a rotating shaft over 360 degrees. The rotation angle detecting device includes two magnetic shafts. The two magnetic shafts engage with the rotating shaft independently from each other. The rotation angles of the magnetic shafts are detected by respective magnetic sensor elements. Each of the magnetic sensor elements outputs a signal having a phase angle different from each other. A signal processor calculates a rotation angle over 360 degrees based on a difference in the phase angle of the signals from the magnetic sensor elements.
The above-described rotation angle detecting device can detect a rotation angle over 360 degrees. However, in the rotation angle detecting device, two sets of gear mechanisms, magnets, and the magnetic sensor elements are arranged around the rotating shafts. Thus, the number of parts and a dimension of the rotation angle detecting device increase and a production cost increase.
US 2007/0194786 A (corresponding to JP-A-2007-256250) by the inventors discloses a rotation angle detecting device that includes a first gear and a second gear. The first gear is fixed to a rotating shaft. The second gear engages with the first gear and rotates around a magnetic sensor element. At each tooth of the second gear, a male screw portion is provided. The male screw portion is screwed with a female screw surface provided at a screw receiver. The screw receiver has a semi-cylindrical shape and is in contact with an outer periphery of the second gear. The second gear rotates while moving in an axial direction biased by the screw receiver. A magnetized body disposed in the second gear provides an inclining magnetic flux density that continuously changes in the axial direction of the second gear. The magnetic sensor element is disposed on an axis of the second gear. A magnetic flux density that penetrates the magnetic sensor element continuously changes in accordance with the rotation angle of the second gear. As a result, the rotating angle detecting device can calculate a rotation angle over 360 degrees based on a difference in phase angle and strengths of two signals from the magnetic sensor element.
In the rotation angle detecting device disclosed in US 2007/0194786 A, the second gear and the screw receiver are screwed with each other and configurate a screw mechanism. The male screw portion of the second gear is only formed at the teeth of the second gear. Thus, the male screw portion of the second gear has a small contact surface and is applied with a high contact pressure compared with a general male screw portion formed throughout the whole circumference. Thus, the male screw portion of the second gear is subject to wear. The configuration of the rotation angle detecting device disclosed in US 2007/0194786 A is simple compared with the rotation angle detecting device disclosed in US 2005/0030011 A. However, the second gear disposed around the magnetic sensor element engages with both of first gear of a steering shaft and the screw receiver. Thus, the second gear is subject to wear and is subject to receive an effect of chattering.